This invention relates to vehicle wheel alignment systems, and in particular to improved sensors in a vehicle wheel alignment system.
Various systems have been designed to determine vehicle wheel alignment angles such as camber, caster, steering axis inclination (SAI), and toe. These systems conventionally employ an optical emitter and an associated optical receiver mounted on cooperative pairs of transducer, as is described in U.S. Pat. No. 5,488,471 to McClenahan et al. A sensor head emits a signal which is transmitted to the receiver of the associated sensor head of the pair. The receiver converts this signal into a value which is indicative of the corresponding toe angle of the vehicle. Thus, wheel alignment sensors have operated using essentially cooperative pairs of emitters and detectors wherein the detector, mounted on one wheel, actively senses the angle of the line of sight to a corresponding emitter mounted on an adjacent wheel. The signals presently used in these sensor heads is an electromagnetic signal in the visual or infrared range (hereinafter, referred to as light). The light impinges upon a sensing device in the receiver whose output is representative of the measured angle. Currently, photodiodes, as set forth in U.S. Pat. No. 4,302,104, which is incorporated herein by reference, and linear array type charge coupled devices (CCDs), as set forth in U.S. Pat. No. 5,018,853, are used as the receiver.
Although the individual detector sensor assembly construction and operation can vary, a conventional prior art example is depicted in FIGS. 1-4. The view of FIG. 1 is of a passenger vehicle 14 which will serve to illustrate the utility of the presently preferred embodiment of the invention. As seen from the left side, the left steerable wheel 15L is shown in association with one form of an instrument support 16 adapted to grip the flange of the wheel rim. The support 16 carries a pivotal housing 17 the axis of which is substantially centerable to the spindle axis (not shown) on which the wheel 15L rotates. A bracket 18 is hung from the housing 17 so it may assume a substantially vertical position even though the wheel 15L is jacked up so it may rotate. At times, with the wheel 15L resting on its support, it may be desirable to secure the bracket 18 against pendulous movement by tightening up on a knob 19 (FIG. 2). The bracket 18, in addition to the housing 17, carries a support arm 20 which extends forwardly of the housing 18 to clear the tread of wheel 15L and be in position so that its end portion may be used for supporting an instrument device 21L. The support arm 20, or some associated part of the assembly, is usually provided with a spirit level (not shown) for purposes of locating the arm in substantially horizontal position, which position is retained by tightening up on the knob 19.
Further shown in FIG. 1 is the vehicle non-steerable wheel 22L to be provided with an instrument support 16 which is identical to the support attached to the steerable wheel 15L. The several parts are designated by similar reference numerals and need not be described again. It is particularly important to observe that the support 16 at the left steerable wheel 15L carries an instrument 23L and the support 16 for the left non-steerable wheel 22L carries a companion instrument 24L. These instruments 23L and 24L are made up of cooperating components which are intended to function with each other in a manner set forth in U.S. Pat. No. 4,319,838 to Grossman, et al.
FIG. 2 shows a schematic plan view of all vehicle wheels, such as those at the left side seen in FIG. 1, and companion right side wheels 15R and 22R. The wheels at the left side are distinguished by adding the suffix "L", and those at the right side are distinguished by the suffix "R". However, each wheel 15R and 22R is provided with an instrument support 16 having the construction generally described above. Also, the support 16 on steerable wheel 15R has a support arm 20 which carries an instrument 21R to cooperate with the left side instrument 21L. In addition, the support 16 at the non-steerable wheel 22R carries an instrument 24R to cooperate with an instrument 23R carried by the support 16 at the steerable wheel 15R. These instruments 21L and 21R, as well as instruments 23R and 24R, cooperate with each other and are made up of components operating in a manner described in the '838 Grossman et al. patent.
In view of FIG. 2, the instruments 21L and 23L are in communication into a console assembly 25, such as by cables 26L and 26R, or by a conventional wireless communications system, and in like manner, the instruments 24L and 24R are in communication with console 25. Signal processing and alignment computation are performed in the console 25 and the results can be displayed by means indicated collectively at 28. More particularly in FIG. 2, the instruments 21L and 21R cooperate with each other in the process of measuring the angles LWT (left wheel toe) and RWT (right wheel toe). For that purpose instrument 21L has radiant energy detector means which is responsive to a source of radiant energy from instrument 21R, and instrument 21R has radiant energy detector means responsive to a source of radiant energy from instrument 21L. The essence of this cooperation is that projectors of radiant energy are disposed to direct beams in criss-cross paths transversely of the vehicle, and which paths have boundaries within the field of vision of the detector means arranged to look at the position from which the beam is projected.
In a like manner, it is indicated in FIG. 2 that instruments 23L and 24L, each containing radiant energy beam projectors and radiant energy detectors, cooperate with each other in the process of measuring the respective angles relative to a vehicle reference axis 30 which is established by a line joining the center points of the axles 31 and 32, which center points are centered between the spacing of the wheel sets 15L and 15R, and 22L and 22R. The angle LFW is formed between the axle 31 and the longitudinal line-of-sight L of the radiant energy beam from the instrument 24L at wheel 22L. The angle LRW is formed between the axle 32 and the longitudinal line-of-sight L of the radiant energy beam from the instrument 23L at wheel 15L. Similarly, the instruments 23R and 24R cooperate with each other for measuring the angles RFW and RRW by the criss-crossing of the radiant energy beams depicted by the dash line R representing the longitudinal line-of-sight between the detector means in the instruments 23R and 24R. In the example seen in FIG. 2, the wheels 15L and 15R have planes of rotation PR which are substantially perpendicular to the axle 31, while the planes of rotation PR of the wheels 22L and 22R are also substantially perpendicular to axis 33. This arrangement shows that the angles LWT and RWT are substantially ninety degrees (90.degree.) since it is presumed that the instrument support arms 20 are substantially parallel to the planes of rotation PR for wheels 15L and 15R. However, it is shown in FIG. 2 that the tread spacing for wheels 22L and 22R is greater than for the tread spacing of the wheels 15L and 15R. In addition, wheel 22L is toed out relative to the reference axis 30 while wheel 22R is toed in relative to the same axis 30. The angular positions for the respective wheels 15L, 15R, 22L and 22R are arbitrary for purposes of illustrating the unique advantages of having active instruments at each wheel for measuring wheel position angles from each other and relative to the reference axis 30 for the vehicle.
FIG. 3 is a diagrammatic view of the wheels 15L, 15R, 22L and 22R of the vehicle 14 of FIGS. 1 and 2, but in this view the wheels have been intentionally mis-aligned to illustrate the geometry of wheel alignment investigation using the foregoing principle instrumentation. The instruments are generally shown and designated by the reference characters appearing in FIG. 2, and the alignment is calculated with reference to a geometric center line 30 (FIGS. 2) of the vehicle. It is necessary to understand that there is a line-of-sight T between the instruments 21L and 21R which represents the radiant energy beam path from the respective instruments 21L and 21R. The line-of-sight may not be the center of the beam, but the beam has a sufficient spread or fan to be seen by the opposing beam sensors. Normally the wheels will not be so far out of alignment as is depicted in FIG. 3 that the beam will not be seen. In like manner there is a line-of-sight L between the instruments 23L and 24L representing the radiant energy beam path from the respective instruments 23L and 24L. The line-of-sight R between the instruments 23R and 24R depicts the path of the radiant energy beams from those respective instruments. There are construction lines on the drawing of FIG. 4 to assist in visualizing the angles to be investigated, such as the dash lines which are parallel to the geometric center line 30, and act as a reference for the angles.
The angles indicated in FIG. 3 are shown in tabular form with reference to the position of the beam projectors, and beam sensors used to determine those angles.
PROJECTOR SENSOR LOCATION LOCATION MEASURED ANGLE Right front Left front Left cross LC toe arm toe arm Left front Right front Right cross RC toe arm toe arm Left rear wheel Left front wheel Left front longitudinal LF Right rear wheel Right front wheel Right front longitudinal RF Left front wheel Left rear wheel Left rear longitudinal LR Right front wheel Right rear wheel Right rear longitudinal RR
The following computations relative to the geometric reference line 30 are worked out for the several angles pertinent to the alignment determination, as follows:
 ANGLES COMPUTED ALGORITHM LFT (left front toe) 1/2(LC + RC + LF - RF) RFT (right front toe) 1/2(LC + RC - LF + RF) TFT (total front toe) LFT + RFT = LC + RC SB (set back) 1/2(RC - LC + LF - RF) LRT (left rear toe) LFT - LF + LR = (LC + RC - LF - RF) + LR RRT (right rear toe) RFT - RF + RR = 1/2(LC + RC - LF - RF) + RR TRT (total rear toe) LRT + RRT = LC + RC - LF - RF + LR + RR TL (thrust line) 1/2(LRT - RRT) = 1/2(LR - RR) LFTTH (left front toe LFT - TL relative to thrust line) RFTTH (right front toe RFT + TL relative to thrust line)
FIGS. 4A and 4B are perspective and top views, respectively, illustrating a prior art linear CCD angle detector 166 which may be utilized with the present invention. Sensor 166 comprises light source 168 and optical bench 170. Radiation source 168 is coupled to one head unit, 118L for example, and optical bench 170 is mounted to another 155 head unit, 18R for example.
Optical bench 170 includes linear CCD 172 and frame 174. Frame 174 includes a mask 176 which defines a slit 178. Slit 178 may comprise a cylindrical lens, and a filter (not shown) may be placed in front of CCD 172 to reduce interference from stray light sources. Slit 178 is at a right angle to linear CCD 172, allowing a portion of the light from light source 168 to fall upon linear CCD 172. The remaining portion of the light from light source 168 directed at linear CCD 172 is blocked by mask 176. As seen in FIG. 5, the angle A at which the light passing through the slit 178 deviates from an axis perpendicular to the CCD 172 corresponds to a distance D along the CCD 172 at which the light will be detected, and accordingly, can be calculated through conventional algorithms once the point of illumination on the CCD 172 is known, yielding the relationship between the light source 1687 and the CCD 172. A suitable linear CCD 172 is TCD 102D available through Toshiba, 7300 Metro Boulevard, Edina, Minn. 55435.
FIG. 4A shows the relationship between light source 168 and optical bench 170 at an angle of 0 degrees. FIG. 4B shows the relationship of light source 168 and optical bench 170 when light source 168 is not in alignment with optical bench 170.
Typically, six angle sensors 166 in the horizontal plane are used to measure the alignment of the wheels of an automotive vehicle. Angle sensor 166 can use infrared or visible light sensors and sources. The multi-element linear CCD 172 comprises a row of 2048 pixels and is used to sense light from light source 168. Slit 178 and mask 176 allow only a portion of the 2,048 pixels to be illuminated by light source 168. Typically, the image will illuminate about 20 to 22 pixel elements of linear CCD 172. However, the width of the image projected on linear CCD 172 may range from about 2 pixel elements to about 80 pixel elements.
As optical bench 170 is rotated and the angle between optical bench 170 and light source 168 is changed, the line of light 179 admitted by slit 178 moves across the pixels of linear CCD 172. At zero degrees, pixel elements near the center of linear CCD 172 are illuminated. As the bench is rotated, elements farther from the center of the sensor are illuminated. The angle of rotation A is found by determining which of the pixel elements of linear CCD 172 are illuminated and calculating how far the angle is from zero degrees, as is seen in FIG. 5.
Slit length determines the range of the allowed tilt of the sensor in an axis perpendicular to the axis of measurement (allowed camber angle) while measuring toe or allowed pitch angle while measuring toe with track sensors. A range of plus or minus 12 degrees is obtained by making the length of slit 178 about equal to the length of linear CCD 172. This should be sufficient for most alignment needs. If the distance between slit 178 and linear CCD 172 is changed from 25/8 inches, the length of slit 178 should also be changed to maintain a range of plus or minus 12 degrees. For example, if the distance between slit 178 and linear CCD 172 is doubled, the length of slit 178 must also be doubled.
The purpose of linear CCD 172 is to convert light energy from light source 168 into electrical energy and provide an output representing an image. Linear CCD 172 uses a linear array of about 2,048 photo-sensitive cells (pixels) which collect light for a controlled period of time (the exposure time), and provide a serial output of data from the pixels. This output is essentially a "snapshot" or "picture" of the scene to which linear CCD 172 was exposed. An algorithm is employed to locate the image of the slit which falls on linear CCD 172 due to light source 168 and the angular relationship between light source 168 and optical bench 170 is calculated.
Equipment of this general type and using the apparatus and methods enumerated above has been used world-wide for many years. Such equipment is capable of determining the camber, caster, and pointing or "toe" alignment angles of the wheels relative to one or more appropriate reference axes, and is sufficient to allow proper adjustment of the alignment so as to reduce tire wear and provide for safe handling. It is believed, however, that such equipment could be improved in terms of both cost and reliability. Moreover, checking the calibration of presently available systems is not a particularly accurate process. Such calibration checks are particularly unsuited for checking camber calibration and even for toe calibration those checks do not typically identify the sensor transducer which may be out of calibration.